Post-1980 Proxies #4: Wilmking et al. [2004]

Wilmking et al. carried out a comprehensive sampling of 1558 white spruce trees at 13 treeline sites in the Brooks Range and Alaska Range. The date of the sampling is not stated but seems to be between 2000 and 2003 and includes the warm 1990s. These findings cover the same species as were primarily used in Jacoby and d’Arrigo’s northern treeline temperature reconstruction, used in multiproxy reconstructions Jones et al [1998], Jones and Mann [2004] as well as MBH98. They found that 40% of the trees had a negative growth response to increased temperatures, 36% had a positive response and the balance had no significant response. They state:

Without accounting for these opposite responses and temperature thresholds, climate reconstructions based on ring width will miscalibrate past climate.

Jacoby and d’Arrigo had hypothesized a linear relationship between ring width and temperature, an assumption embedded in MBH98 as a condition for a valid proxy. Instead, Wilmking et al. report:

Our findings of both positive and negative growth responses to climate warming at treeline challenge the widespread assumption that arctic treeline trees grow better with warming climate. High mean temperatures in July decreased the growth of 40% of white spruce at treeline areas in Alaska, whereas warm springs enhance growth of additional 36% of trees and 24% show no significant correlation with climate.

They go on to state:

The finding of highly significant negative relationships between July temperature and radial growth as the most common climate signal present at treeline and near treeline sites in Alaska is quite surprising, and apparently not consistent with much published literature… Here we report that the most frequent growth response to increasing temperatures is negative at the northern and altitudinal tree limit in Alaska. Not only single trees, but large populations of trees at every site show reduced growth with warming temperatures in North America (Garfinkel & Brubaker, 1980; Jacoby & D’Arrigo, 1989; Jacoby et al., 1996; Overpeck et al., 1997; Lloyd & Fastie, 2003).

As I mentioned in my earlier posting on D’Arrigo et al. re the TTHH site, the breakdown or non-existence of a linear relationship between temperature and ring width poses serious problems for climate reconstructions, as it is impossible to tell whether a narrow ring represents a cold or warm leg of the upside-down U relationship.

Update Mar 2007:

The Brooks Range sites are located here:

ITRDB identification for all Brooks Range sites but one are as follows:

4 Comments

Has this individual study been confirmed by other results or is this a singular result up to the present? It would become an interesting study once it is clear it isn’t a singular result without independent confirmation from other studies.

If anything the criticism I have of scientific climate skeptics would be the tendency to take a single study and present it as a radical revolutionary contribution. I am a skeptic myself though and I would like to see policy better supported by good scientific data as well, so this is an interesting result, but not enough to start breaking down all tree ring studies just yet.

If it helps, Florens, I would agree with you. Replication of scientific results by other teams is imperative for a result to be considered as “robust”. But, I’d think you’d agree, the research of Wilmking et al., is suggestive. That is all.

Have you corresponded with them? What is the dialogue between these type of guys and the linear guys? how can we tell if this is an isolated observation? If there are ways to ensure that linearity works?